declare @DateOfBirth datetime
set @DateOfBirth = '1978-01-26 00:00:00.000'

select @DateOfBirth

select DATEDIFF(yy, @DateOfBirth, GETDATE())  

select FLOOR(CONVERT(decimal(9, 2), DATEDIFF(d, @DateOfBirth, GETDATE())) / 365.0)  

select DATEDIFF(yy, @DateOfBirth, GETDATE()) - CASE WHEN DATEPART(m, @DateOfBirth) >= DATEPART(m, GETDATE()) AND DATEPART(d, @DateOfBirth) >= DATEPART(d, GETDATE()) THEN 0 ELSE 1 END 

declare @DateOfBirth datetime
declare @Today datetime

select @DateOfBirth = '2004/03/01'
select @Today = '2008/02/29'

select
  DATEDIFF(yy, @DateOfBirth, @today) 
select
  FLOOR(CONVERT(decimal(9, 2), DATEDIFF(d, @DateOfBirth, @Today)) / 365.0) 
select
  DATEDIFF(yy, @DateOfBirth, @today) 
  - CASE 
    WHEN DATEPART(m, @DateOfBirth) >= DATEPART(m, @Today) AND DATEPART(d, @DateOfBirth) >= DATEPART(d, @Today) THEN 
      0 
    ELSE 
      1 
    END 
select
  DATEDIFF(yy, @DateOfBirth, @today) 
  - CASE 
    WHEN DATEPART(m, @Today) > DATEPART(m, @DateOfBirth)
       OR (DATEPART(m, @Today) = DATEPART(m, @DateOfBirth)
           AND DATEPART(d, @Today) >= DATEPART(d, @DateOfBirth)) THEN 
      0 
    ELSE 
      1 
    END